In amyotrophic lateral sclerosis (ALS), a fatal progressive neurodegenerative disorder, motor neurons selectively die, for reasons that are incompletely understood. This remarkable selective vulnerability provides an important clue to the mechanism of this devastating disease. Evidence suggests that ALS involves glutamate excitotoxicity mediated by AMPA receptors, and our work has begun to elucidate the basis for the selectivity for motor neurons of this mechanism. We showed that vulnerability to excitotoxicity of cultured spinal motor neurons correlates not with their expression of AMPA receptors having a particularly high permeability to Ca2+ or a particularly weak degree of desensitization, but rather with expression of AMPA receptors at a very high surface density. This property, expression of a high density of functional AMPA receptors, appears to be sufficient to explain the in vitro selective vulnerability of spinal motor neurons. More generally, the hypothesis serving as a theme of the entire project is that motor neuron selective vulnerability in ALS may be largely explained by the unique features of their glutamate receptor expression. The present proposal will extend the work of the initial funding period to address three important issues relating to this hypothesis. Specific aim 1 will define the physiological and molecular characteristics of AMPA receptors expressed by mature motor neurons in the tissue environment of the spinal cord, using patch-clamp and molecular techniques applied to motor neurons in acute spinal cord slices from mature rats. Specific aim 2 will determine whether upper motor neurons, which form the corticospinal tract, also exhibit a pattern of glutamate receptor expression that explains their selective vulnerability, whether it be AMPA receptors or NMDA receptors that primarily mediate injury in these cells.